A jailed member of the punk band Pussy Riot has launched a hunger strike to protest against slave-like living conditions and an alleged death threat from the deputy head of the prison where she is serving her two-year sentence.

Nadezhda Tolokonnikova has written a long letter detailing life inside prison colony No 14 in the Russian region of Mordovia, revealing appalling conditions reminiscent of the Soviet Gulag system.

As part of her punishment she has to sew police uniforms, and she writes in her statement that all the prisoners in her sewing division are expected to work 16 or 17 hours a day, starting at 7.30am and not finishing until after midnight.

"In the best case scenario we get four hours of sleep per night," writes Tolokonnikova. "We get a day off once every six weeks. Almost all Sundays are work days. Prisoners are forced to write requests to work on weekends saying it is their own voluntary decision."

Tolokonnikova was convicted of hooliganism motivated by religious hatred for her part in an impromptu Pussy Riot performance in Moscow's largest cathedral in February last year. The group, wearing balaclavas, mimed a song calling on the Virgin Mary to "kick out Putin", and three of them were arrested and later convicted.

Tolokonnikova claims that three weeks ago she complained to the prison's deputy head about the conditions and he informed her that her team would work only eight-hour days from then on. However, he said the team would still have to meet their required output targets – physically impossible in just eight hours – and if they did not, they would all be punished.

"And if they find out that it all happened because of you, then you definitely won't feel bad any more," the Pussy Riot member claims he told her. "Because in the afterlife you don't feel bad."

She says she will refuse food until her concerns are addressed. "As of Monday 23 September I announce that I am on hunger strike," she writes. "This is an extreme method, but I am absolutely certain that it is the only way out of this situation for me. The prison colony's administration refuses to listen to me."

Tolokonnikova portrays a grim life for inmates where, in a practice surviving from the Gulag system, the authorities delegate certain prisoners to do their dirty work for them and keep the other inmates terrorised.

She writes that she has not so far been subjected to physical violence, possibly because the prison authorities are aware of her international fame, but that other women are regularly beaten on the face or kidneys. The beatings are carried out by other inmates, but "never happen without the knowledge and sanction of the prison authorities".

She also writes of a system of informal punishments, such as forcing inmates to spend time outdoors during the harsh winters, or banning women from going to the toilet all day.

Tolokonnikova writes that Mordovia, which recently became the official home of the French actor turned Russian citizen Gérard Depardieu, has the hardest and most terrifying network of prisons, and that all prisoners are scared of being sent there.

A Mordovia prison system spokesman denied all of Tolokonnikova's allegations later on Monday, calling her claims of death threats "absurd" and stating that working days never last longer than eight hours.

"The working day in prison colony No 14 is eight hours, as set out in the labour code," said Gennady Morozov. "The women work until 4.30pm. That is how it always was and there have never been any changes to that."

In May the other jailed member of Pussy Riot, Maria Alyokhina, went on hunger strike for 11 days against conditions in the prison in the Perm region where she is serving her sentence. The prison authorities met many of her demands. Both she and Tolokonnikova are due for release in March next year.